PR libstdc++/87431 optimise valueless_by_exception()

If a std::variant can never get into valueless state then we don't need
to do a runtime check for a valid alternative.

	PR libstdc++/87431
	* include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
	Avoid runtime test when all alternatives are scalars and so cannot
	throw during initialization.

From-SVN: r264574

libstdc++-v3/ChangeLog

+2018-09-25  Jonathan Wakely  <jwakely@redhat.com>
+
+	PR libstdc++/87431
+	* include/std/variant (_Variant_storage<true, _Types...>::_M_valid):
+	Avoid runtime test when all alternatives are scalars and so cannot
+	throw during initialization.
+
 2018-09-25  Gerald Pfeifer  <gerald@pfeifer.com>
 
 	* doc/xml/manual/codecvt.xml: Move link to "UTF-8 and Unicode FAQ"

libstdc++-v3/include/std/variant

@@ -439,6 +439,8 @@ namespace __variant
       constexpr bool
       _M_valid() const noexcept
       {
+	if constexpr ((is_scalar_v<_Types> && ...))
+	  return true;
 	return this->_M_index != __index_type(variant_npos);
       }